During his tenure, Krasner has sought to spearhead criminal justice reform by ending bail payments for low-level offenders, reducing supervision for parolees, and seeking more lenient sentences for certain crimes.[2] Prior to his government service, Krasner was a prolific criminal defense and civil rights attorney and public defender who aggressively pursued police misconduct.[3]

There was no incumbent D.A. in the race as sitting Philadelphia D.A. R. Seth Williams announced that he would not run for reelection in February 2017,[13] and pleaded guilty to federal bribery charges in June 2017, resigning from office. (The acting D.A., Kathleen Martin, chose not to run.)[14] When Krasner announced his candidacy, the president of the Philadelphia police union described his candidacy as "hilarious."[1] Krasner received no major newspaper endorsements.[1]

Krasner won a crowded, seven-way Democratic primary by over 17% on May 16, 2017, defeating former city and federal prosecutor Joe Khan (who placed second); former Philadelphia Managing Director Rich Negrin; former First Assistant District Attorney Tariq El-Shabazz; former prosecutor Michael Untermeyer; former prosecutor Jack O'Neill; and former Municipal Court Judge Teresa Carr Deni.[15][16][17] City officials reported that voter turnout in this race spiked nearly 50 percent, compared to the last contested race for district attorney of Philadelphia, in 2009.[18] The primary was widely seen as the proxy election, as the winner of the primary was the presumptive victor of the general,[11] as registered Democrats outnumbered registered Republicans in Philadelphia by almost 7-to-1.[19][20]

In the November general election, Krasner defeated Republican Beth Grossman, a former assistant district attorney, by nearly a 50% margin.[21]

In his first week in office, Krasner fired 31 prosecutors from the District Attorney's Office, including both junior and career supervisory staff. Up to one-third of the homicide prosecutors in the office were dismissed, and the total number of those fired represented as many as a 10% reduction in the number of Philadelphia assistant district attorneys.[22][23]

In February 2018, Krasner announced that law enforcement would no longer pursue criminal charges against those caught with marijuana possession.[24] That same month, Krasner instructed prosecutors to stop seeking cash bail for those 'accused of some misdemeanors and nonviolent felonies.[25] Krasner said that it was unfair to keep people in detention simply because they could not afford bail.[25] He also announced that the DA's office had filed a lawsuit against a number of pharmaceutical companies for their role in the city's opioid epidemic.[24] Krasner instructed prosecutors to stop charging sex workers who had fewer than three crime convictions.[26]

In March 2018, it was reported that Krasner's staffers were working on creating a sentence review unit–the first of its kind in the country–to review past cases and sentences, and seek re-sentencing in cases where individuals were given unduly harsh punishments.[27]

Also in March 2018, it was reported that Krasner instructed prosecutors to: "Offer shorter prison sentences in plea deals. Decline certain classes of criminal charges. And explain, on the record, why taxpayers should fork over thousands of dollars per year to incarcerate people."[28] He further stated that: "Fiscal responsibility is a justice issue, and it is an urgent justice issue. A dollar spent on incarceration should be worth it. Otherwise, that dollar may be better spent on addiction treatment, on public education, on policing and on other types of activity that make us all safer."[29]

In April 2018, it was reported that some judges had rejected the reduced sentences which Krasner's prosecutors had sought for juveniles which had previously been sentenced to life in prison.[30]

In June 2018, it was reported that Krasner had requested a comprehensive list of police officers who had lied while on duty, used excessive force, racially profiled, or violated civil rights. According to the Philadelphia Inquirer, "The exercise, unprecedented in scope in recent city history, is designed to help prosecutors flag officers with credibility issues early in a case and possibly prevent their testimony".[31]